How gestation reshapes the mouse brain to favor maternal behaviors

2023-10-12 16:00:07
A white mouse nursing its young. BRUNO CAVIGNAUX/BIOSPHOTO

Maternal attachment, this bubble of tenderness. A little miracle that is renewed at each birth, among mammals, when everything goes well. To what extent are maternal behaviors linked to changes in the brains of expectant mothers during pregnancy? In the magazine Science of October 6, a British team provides an element of the answer. In mice, two sex hormones cause, during gestation, brain remodeling which is sufficient to promote these maternal behaviors: returning to the nest, grooming, breastfeeding.

It has long been believed that sex hormones released during parturition play a key role in the onset of maternal care. But, even before birth, estradiol and progesterone act in the brain of the future mother, shows this study in Scienceled by a team from the Francis-Crick Institute, in London.

And here’s how. These two hormones target a small region of the hypothalamus, the medial preoptic area, known for its important role in controlling mating and caring for young. Even more precisely, within this region, these hormones act on a small population of neurons, the “galanin neurons”, known to govern maternal behavior.

In female mice that had never mated, the authors made these neurons insensitive to these two hormones, or to one of the two only – to do this, they selectively inactivated the receptors for these hormones carried by these cells . Then they let half of these females mate. As a result, those who had babies showed a total loss of nurturing behavior.

Responsiveness to signals

But these two hormones operated differently. Estradiol made these neurons more excitable, which strengthened the mothers’ receptivity to signals from their offspring. A change that faded as the little ones grew up.

Progesterone promoted the formation of excitatory synapses on these same neurons. This rewiring persisted, reinforcing maternal behavior long after the offspring had been weaned. “This observation would explain why females of many species exhibit maternal behavior towards offspring that are not their own, once they have experienced motherhood”, comments Margaret McCarthy, from the University of Maryland in Baltimore (United States).

“What is fascinating is to see that the brain prepares itself well before birth for this great change in life”, notes Johannes Kohl, who coordinated this work. Its rewiring results in “a change of priority” : if virgin mice concentrate on mating, mothers must ensure the survival of their young. “To prepare for this future behavioral challenge, there is a window of plasticity in the brain”notes Rachida Ammari, postdoctoral student and first author.

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